This invention relates generally to a method of indicating the relative roughness of a surface. More particularly, though not exclusively, the invention relates to a method of indicating the relative roughness of a surface that has been smoothed by machining, as by forming a plurality of substantially uniform and parallel grooves therein. The invention is particularly useful for indicating the relative roughness of the finished surfaces of projectile casings because excessive roughness can interfere with the proper operation of the projectiles.
The standards of relative roughness, as used herein, are those of the Standard Surface Finish Blocks (S.S.F.B.) standardized by the National Bureau of Standards. The Standard Surface Finish Blocks are designated by numbers, as for example, 32, 64, 125, 250, and 500. The smaller the S.S.F.B. number is, the smoother is the surface it designates.
It has been proposed to indicate the relative roughness of a finished surface by magnifying the surface and visually comparing it with reference surfaces, but such a method is obviously slow and subject to human error. Instruments for measuring the roughness of commercial finishes are usually of the tracer type in which a finely pointed diamond stylus is drawn across the finished surface. The motion of the stylus is highly magnified and recorded by electrical means. While satisfactory for certain materials, the aforementioned instruments may mar the very smooth finish surfaces they are measuring.
Where high-dimensional accuracy and smooth surface finishes are required, bar stock metals, for example, are usually finished by machining, as with a milling machine. These finished surfaces appear smooth to the naked eye, but they actually consist of a plurality of substantially uniform and parallel grooves formed by regularly spaced cuts with a cutting tool.